System and method for integrated circuits with cylindrical gate structures

ABSTRACT

A device and method for integrated circuits with surrounding gate structures are disclosed. The device includes a semiconductor substrate and a fin structure on the semiconductor substrate. The fin structure is doped with a first conductivity type and includes a source region at one distal end and a drain region at the opposite distal end. The device further includes a gate structure overlying a channel region disposed between the source and drain regions of the fin structure. The fin structure has a rectangular cross-sectional bottom portion and an arched cross-sectional top portion. The arched cross-sectional top portion is semi-circular shaped and has a radius that is equal to or smaller than the height of the rectangular cross-sectional bottom portion. The source, drain, and the channel regions each are doped with dopants of the same polarity and the same concentration.

CROSS-REFERENCES TO RELATED APPLICATIONS

This application is a divisional application of U.S. patent application Ser. No. 13/848,707, filed Mar. 21, 2013, which is a continuation-in-part of U.S. application Ser. No. 12/892,881, filed Sep. 28, 2010, which claims priority to Chinese Patent Application No. 200910057965.3, filed Sep. 28, 2009, now issued Chinese Patent No. ZL200910057965.3, by inventors De Yuan Xiao et al. This application also claims priority to Chinese Patent Application No. 201210365185.7, filed Sep. 26, 2012, by inventor De Yuan Xiao. All of the above applications are commonly owned and hereby incorporated by reference for all purposes.

BACKGROUND OF THE INVENTION

The present invention is directed to integrated circuits and their processing for the manufacture of semiconductor devices. More particularly, the invention provides a semiconductor device having a transistor and a method for fabrication thereof. Merely by way of example, the invention has been applied to a field effect transistor (FET) device having a gate-all-around cylindrical (GAAC) nanowire. As an example, the FET is based on a silicon-on-insulator (SOI) wafer substrate and method for the manufacture thereof. But it would be recognized that the invention has a much broader range of applicability.

Integrated circuits or “ICs” have evolved from a handful of interconnected devices fabricated on a single chip of silicon to millions of devices. Current ICs provide performance and complexity far beyond what was originally imagined. In order to achieve improvements in complexity and circuit density (i.e., the number of devices capable of being packed onto a given chip area), the size of the smallest device feature, also known as the device “geometry”, has become smaller with each generation of ICs. Semiconductor devices are now being fabricated with features less than a quarter of a micron across.

Increasing circuit density has not only improved the complexity and performance of ICs but has also provided lower cost parts to the consumer. An IC fabrication facility can cost hundreds of millions, or even billions, of dollars. Each fabrication facility will have a certain throughput of wafers, and each wafer will have a certain number of ICs on it. Therefore, by making the individual devices of an IC smaller, more devices may be fabricated on each wafer, thus increasing the output of the fabrication facility.

However, making devices smaller is very challenging, as each process used in IC fabrication may have certain limits. That is to say, a given process typically only works down to a certain feature size, and then either the process or the device layout needs to be changed. An example of such a limit is that for CMOS processes and technology at reduced gate lengths conventional CMOS device have increasing difficulty in maintaining high drive currents with low off-current leakage and threshold voltage stability as well. The short-channel effect becomes a big hurdle to further scale down the conventional CMOS devices. This results in a degradation of the device performance and determines the limits of miniaturization. Over the past, various conventional techniques have been developed to overcome the abovementioned limits. Unfortunately, these techniques have often been inadequate.

Therefore, it is desirable to have an improved gate structure for MOS devices and processes thereof.

BRIEF SUMMARY OF THE INVENTION

The present invention is directed to integrated circuits and their processing for the manufacture of semiconductor devices. More particularly, the invention provides a semiconductor device having a transistor and a method for fabrication thereof. Merely by way of example, the invention has been applied to a field effect transistor (FET) device having a fin structure. As an example, the FET is based on a silicon-on-insulator (SOI) wafer substrate and method for the manufacture thereof. But it would be recognized that the invention has a much broader range of applicability.

As used herein, the terms “wafer” and “substrate” are to be understood as including silicon-on-insulator (SOI) or silicon-on-sapphire (SOS) technology, doped and undoped semiconductors, epitaxial layers of silicon supported by a base semiconductor foundation, and other semiconductor structures. Furthermore, when reference is made to a “wafer” or “substrate” in the following description, previous process may have been utilized to form regions or junctions in the base semiconductor structure or foundation. In addition, the semiconductor need not be silicon-based, but could be based on silicon-germanium, germanium, or gallium arsenide.

A specific embodiment of the invention provides a transistor device having a fin structure. The transistor device includes a semiconductor substrate, a fin structure protruding from the semiconductor substrate and extending in a first direction. The fin structure includes a first type impurity and has a source region disposed at one end and a drain region disposed at the opposite end, and a channel region disposed between the source and drain regions. The transistor device further includes a gate structure defining the channel region. The gate structure comprises a dielectric layer and a conductive material overlying the dielectric layer. Additionally, the fin structure has a rectangular cross-section bottom portion and an arched cross-section top portion. The source, drain, and channel regions each are doped with dopants of a same polarity and a same doping concentration. In an embodiment, the doping concentration is about 2×10¹⁹ atoms/cm³ or greater.

In an embodiment, the gate structure may include spacers disposed on side surfaces of the dielectric layer and the conductive material to protect the gate structure from damage in subsequent processes.

In an embodiment of the invention, the semiconductor substrate comprises a silicon substrate and a silicon oxide layer overlying the silicon substrate. In one embodiment, the first impurity is an n-type impurity when the transistor device is an NMOS transistor. In another embodiment, the first impurity is a p-type impurity when the transistor device is a PMOS transistor.

In an embodiment, the semiconductor substrate comprises a silicon material having a region that is in contact with the bottom portion of the fin structure. The region includes a second-type impurity.

In an embodiment, the transistor device is an NMOS transistor, the first type impurity is an n-type impurity, and the second-type impurity of the region in contact with the bottom of the fin structure is a p-type impurity. In another embodiment, the transistor device is a PMOS transistor, the first type impurity is a p-type impurity, and the second-type impurity of the region in contact with the bottom of the fin structure is an n-type impurity.

In an embodiment, the arched top portion of the fin structure is characterized by a half circle having a radius, and the rectangular cross-section bottom portion is characterized by a height that is equal to or greater than the radius of the half circle.

Embodiments of the invention also provide a method for fabricating a semiconductor device. The method includes providing a semiconductor substrate and forming an elongated semiconductor structure overlying a region of the semiconductor substrate along a first direction. The elongated semiconductor structure has a rectangular cross-section with a width and a height. The method further includes etching the elongated semiconductor structure to provide an arched top portion. Additionally, the method includes implanting dopants of a first-type impurity in the elongated semiconductor structure, forming a dielectric layer on the elongated semiconductor structure, and a conductive material overlying the dielectric layer. The method further includes masking and etching the conductive material and the dielectric layer to form a channel region.

In an embodiment, the method further includes implanting dopants of a second-type impurity in the region of the semiconductor substrate prior to forming the elongated semiconductor structure.

In an embodiment, the method further includes etching around the elongated semiconductor structure to form a trench and filling the trench with an insulating material. In an embodiment, the forming of the insulating trench is prior to the implanting of dopants of the first-type impurity in the elongated semiconductor structure.

In an embodiment, the elongated semiconductor structure comprises silicon, germanium, silicon-germanium, or other III or V group semiconductor materials.

In an embodiment, forming the elongated semiconductor structure comprises oxidizing and removing the oxidized portions of the elongated semiconductor structure. The removal of the oxidized portions may include a diluted hydrofluoric acid.

In an embodiment, the method further includes hydrogen annealing after the removal of the oxidized portion of the elongated semiconductor structure. The hydrogen annealing may be performed at a temperature range from about 1000° C. to about 1200° C. for a time period of about 5 minutes to about 30 minutes. In an embodiment, the method further includes forming a first spacer and a second spacer. The first and second spacers each are in contact with opposite surfaces the gate structure. In an embodiment, the spacers include Oxide/Nitride/Oxide (ONO) layers.

It is to be understood that the present invention has a wide range of applicability. For example, certain embodiments are compatible with existing system and processes. Depending upon application, various embodiments of the present invention may be readily modified.

Various additional features and advantages of embodiments of the present invention can be more fully appreciated with reference to the detailed description and accompanying drawings that follow.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a simplified diagram showing schematic gate cross-sections of a series conventional multi-gate devices and a gate all-around cylindrical (GAAC) device according to an embodiment of the present invention;

FIG. 2 is a simplified top-view of a surface region of an insulation layer on a substrate, the surface region comprising a first region positioned along an axis of B-B′ and a second region crossing the first region along a substantially perpendicular axis of A-A′ are defined, according to an embodiment of the present invention;

FIG. 3A is a simplified perspective view of a device structure in accordance with an embodiment of the invention;

FIG. 3B is a simplified cross-sectional view cutting along the A-A′ plane of FIG. 3A;

FIG. 3C is a simplified cross-sectional view cutting along the B-B′ plane of FIG. 3A;

FIG. 4A is a simplified diagram showing a cylindrical semiconductor wire divided by a source region, a drain region, and a gate surrounding the channel region (not shown) in the middle section according to an embodiment of the present invention.

FIG. 4B is a simplified diagram showing a cross-section view of the gate region in FIG. 4A according to an embodiment of the present invention;

FIG. 4C is a simplified diagram showing a plot of electric field across the channel region with a length L in FIG. 4B according to another embodiment of the present invention;

FIG. 5A is a simplified diagram showing drain current as a function of drain voltage for a gate-all-around cylindrical nanowire finFET device at the different gate bias voltages according to an embodiment of the present invention;

FIG. 5B is a simplified diagram showing drain current as a function of gate voltage for a gate-all-around cylindrical nanowire finFET device with a drain voltage of 0.1V according to an embodiment of the present invention;

FIG. 5C is a simplified diagram showing drain current as a function of drain voltage for a gate-all-around cylindrical nanowire finFET device with different gate separation widths;

FIG. 5D is a simplified diagram comparing drain current as a function of gate voltage for a gate-all-around cylindrical nanowire finFET with that of Tri-gate, Pi-gate, and Omega-gate devices.

FIG. 6 is a simplified method for fabricating a semiconductor device having a gate-all-around cylindrical nanowire architecture according to an embodiment of the present invention;

FIGS. 7A, 8A, 9A, 10A, 11A, 12A, 13A, and 14A are simplified diagrams showing perspective views illustrating process steps in accordance with the embodiment of the invention shown in FIG. 6;

FIGS. 7B, 8B, 9B, 10B, 11B, 12B, 13B, and 14B are simplified cross-sectional views taken along the A-A′ plane of FIGS. 7A, 8A, 9A, 10A, 11A, 12A, 13A, and 14A respectively;

FIGS. 8C, 9C, 10C, 11C, 12C, 13C, and 14C are simplified cross-sectional views taken along the B-B′ plane of FIGS. 8A, 9A, 10A, 11A, 12A, 13A, and 14A respectively;

FIG. 15A is a simplified perspective view of an integrated circuit unit showing three contact pads for a gate-all-around cylindrical nanowire finFET device within an inner dielectric layer according to an embodiment of the present invention;

FIG. 15B is a simplified cross-sectional view cutting along the B-B′ plane of FIG. 15A;

FIG. 16 is a simplified perspective view of a conventional finFET device;

FIG. 17 is a simplified perspective view of a transistor device according to an embodiment of the present invention;

FIGS. 18 and 19 are simplified cross-sectional views of the transistor device in FIG. 17;

FIGS. 20 through 22 are cross-sectional views of a channel region of a transistor device under different operation conditions according to embodiments of the present invention;

FIG. 23 is a simplified perspective view of a transistor device according to another embodiment of the present invention;

FIGS. 24 and 25 are simplified cross-sectional views of the transistor device in FIG. 23;

FIG. 26 is a simplified flow diagram illustrating a method for fabrication one or more transistor devices having a semi-circular window shaped fin structure according to an embodiment of the present invention;

FIGS. 27 through 30 illustrate the method for fabrication one or more transistor devices of FIG. 26;

FIGS. 31 through 34 illustrate a sequence of processes for fabricating one or more transistor devices according to another embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention is directed to integrated circuits and their processing for the manufacture of semiconductor devices. More particularly, the invention provides a semiconductor device having a transistor and a method for fabrication thereof. Merely by way of example, the invention has been applied to a field effect transistor (FET) device having a gate-all-around cylindrical (GAAC) nanowire. As an example, the FET is based on a silicon-on-insulator (SOI) wafer substrate and method for the manufacture thereof. But it would be recognized that the invention has a much broader range of applicability.

As discuss above, various conventional techniques have been developed for FET devices. For example, to fabricate devices beyond current scaling limits, alternative gate stack materials, band engineering methods, and alternative transistor structures are all explored rigorously. It has been studied that for FET the single gate full-depleted single-gate SG-FD device cannot be scaled as short as a channel length as dual-gate (DG) fin type FET device, which is due to the fact that there is no field confinement with a single gate conducting plane. Unfortunately, the dual-gate finFET does not confine the field completely. Naturally multi-gate finFET devices are developed as a better choice in terms of both low off-current leakage and short-channel effect immunity. The multi-gate finFET device also is feasible to scale down with high speed, low power dissipation, low short channel effects, etc.

FIG. 1 is a simplified diagram showing a series of gate structure cross-sections for various evolved multi-gate devices in terms of their equivalent number of gates. The diagrams included in the dashed box belong to the conventional devices starting with the SG device. First, beyond the DG device, it comes with Tri-gate (TG) device with a gate region covering three sides of the channel region to improve the field confinement. Further, Pi-gate (PG) or Omega-gate (OG) device (with 3+ gates) extends the gate region of the tri-gate device trying to reduce the opening gap of the gate region thus reducing the off-current leakage.

More recently, gate-all-around (GAA) device has been developed in which the gate region surrounds the channel region completely without leaving a gap as other previous multi-gate device. However, due to the process limitation, the channel regions of these GAA type devices are in rectangular shape. In other words, the GAA device basically is an equivalent 4-gate type device or named as a gate-all-around rectangular (GAAR) device. For making a GAAR type device, there are some process difficulties existing for forming a channel bridge due to the relative large width of the channel structure. For example the well-established planar CMOS process technology cannot be applied to form the required GAAR structure in some proposed designs. Additionally, electrical field in the rectangular channel is still not uniform due to the unavoidable corner effect. Therefore, as shown in the last schematic diagram outside the dashed box in FIG. 1, according to an embodiment of the present invention, a device with essentially infinite number of gates surrounding a cylindrical channel is provided to reduce the problems of the conventional multi-gate devices. It is to be understood that the device as shown in FIG. 1 is merely an example, which may be altered and/or modified, which should not limit the scope of claims.

FIG. 2 shows a top view of a surface of an oxide layer overlaying a substrate according to an embodiment of the present invention. This diagram is merely an example, which should not unduly limit the scope of the claims herein. One of ordinary skill in the art would recognize many variations, alternatives, and modifications. For the convenience of description, the surface region 20 is divided by a first region 1 along and around an axis B-B′, a second region 2 along and around an axis A-A′, and the rest surface of oxide 20. According to an embodiment of the present invention, the axis A-A′ is substantially perpendicular to the axis B-B′. The region 1 crosses and overlaps with the region 2. The semiconductor device according to an embodiment the present invention is structured in both the region 1 and the region 2 and interposed in vertical dimension as shown below.

FIGS. 3A, 3B, and 3C show a simplified diagram for a gate-all-around cylindrical (GAAC) nanowire finFET device 1000 according to an embodiment of the present invention. These diagrams are merely examples, which should not unduly limit the scope of the claims. One of the ordinary skill in the art should recognize many variations, alternatives, and modifications. FIG. 3A provides a perspective view of the device 1000. FIG. 3B illustrates a cross-section view cutting along an A-A′ plane of the FIG. 3A. The A-A′ plane as shown is a vertical plane along the axis A-A′ in FIG. 2. FIG. 3C illustrates a cross-section view cutting along a B-B′ plane of the FIG. 3A. The B-B′ plane is a vertical plane along the axis B-B′ in FIG. 2. The device 1000 includes the following components: a substrate region 10, an insulation layer 20, undercut structure 21 and 22, a channel region 30, a source region 31, a drain region 32, a gate region 60, a gate dielectric layer 50, insulation spacer regions 71 and 72.

Although the above has been shown using a specific group of components for the device 1000, there can be many alternatives, modifications, and variations. For example, some of the components may be expanded and/or combined. Other components may be inserted to those noted above. Depending upon the embodiment, the arrangement of components may be interchanged with others replaced. For example, the device 1000 may be an N-type GAAC finFET. In another example, the device 1000 may be a P-type GAAC finFET. Further details of these components are found throughout the present specification and more particularly below.

Referring to FIG. 3A, the components mentioned above are formed on the insulation layer 20 overlaying a bottom substrate 10 according to one embodiment of the invention. For example, the insulation layer 20 is the buried oxide layer of an SOI wafer substrate. In another example, a layer of semiconductor, i.e., the SOI layer, overlays the insulation layer 20 or the buried oxide layer. The threshold-adjustment ion implantation is performed in the SOI layer with either n-type dopants or p-type dopants. In one example, the n-type dopant is Boron ion. In another example, the p-type dopant is Arsenic ion. In yet another example, the dopant concentration is in a range from 10¹⁸ to 10¹⁹ cm⁻³.

Referring to FIGS. 2, 3A, 3B, and 3C, according to an embodiment of the present invention, the device 1000 includes a wire pattern formed by patterning of the SOI layer on the surface of insulation layer 20 in the region 1 along the axis B-B′. As shown in FIG. 3C, the wire pattern includes a source and/or drain regions 31 and 32 disposed in the two opposite end-sections and a channel region 30 in the middle section. As an example, the wire pattern is symmetrical, and thus drain and source regions may be flipped. In one example, the wire pattern through its whole length including two end-sections and middle section is associated with a width in a range of 4 nm to 50 nm. As an example, the wire is referred to as “nanowire” through various parts of the application, and should not unduly limit the scope of claims.

Referring to FIGS. 3A and 3C, according to one embodiment of the present invention, the source region 31 and the drain region 32 are formed through heavily doping the two end-sections with a dopant polarity that is opposite to the channel region 30. For example, the channel region has its doping substantially the same as the SOI layer. In a specific instance, the source/drain regions 31 and 32 are doped heavily with N⁺ implant as the channel region 30 is doped as p-type. In another example, the source/drain regions 31 and 32 are doped heavily with P⁺ implant as the channel region 30 is doped as n-type.

As shown in FIGS. 3A, 3B, and 3C according to an embodiment of the present invention, the source region 31 and the drain region 32 are in contact with the insulation layer 20 through undercut structure 21 and 22, respectively. The undercut structure 21 or 22 has a same dielectric material as the insulation layer 20. The channel region 30 is characterized by a cylindrical shape with a specific length 35 as shown. According to various embodiment, the channel region 30 may not be in directly in contact with the underneath insulation layer 20. For example, the channel length 35 of the cylindrical channel region 30 is surrounded by a gate insulation layer 50. In one embodiment as shown in FIGS. 3B and 3C the cylindrical shape of the channel region 30 having the channel length 35 surrounded by the gate dielectric layer 50 is preserved. In another embodiment, the gate dielectric layer 50 can be made of a thin layer of material with a high dielectric constant (e.g., defined by high-k≧8) and a large bandgap. In an example embodiment, the gate dielectric layer 50 is hafnium oxide H_(f)O with a k value as high as 34. In another example embodiment, the gate dielectric layer 50 may be grown by atomic layer deposition (ALD) and has a thickness in a range between 1 nm and 3 nm.

According to some embodiments of the present invention, as shown in FIGS. 2, 3A, 3B, and 3C, the device 1000 further includes a gate region 60 that is formed on the insulation layer 20 in the region 2 along the axis A-A′. In one embodiment, the gate region is positioned along the axis A-A′ direction to cross the wire pattern in the channel region 30 and confined along the B-B′ direction by the channel length 35. The direction of the axis A-A′ is substantially perpendicular to the axis B-B′. In another embodiment, referring to FIGS. 3B and 3C, the gate region 60 entirely surrounds the cylindrical shaped channel region 30 with the gate dielectric layer 50 interposed. In one example, the gate region 60 is in-situ N⁺ heavily doped poly-silicon layer with a thickness ranging from 100 nm to 500 nm. In another example, the conductive material of the gate region 60 is made from a metal layer with a thickness ranging from 10 nm to 200 nm deposited by a chemical vapor deposition (CVD) method. It is to be understood that embodiments of the present invention may be implemented in various ways. According to an embodiment of the present invention, the device 1000 includes a gate-all-around cylindrical nanowire architecture with a gate length that is about the same of the channel length 35.

As shown in FIGS. 3A and 3C, the gate region 60 is separated from the wire pattern in the channel region 30 by the gate dielectric layer 50. The gate region 60 is also not in direct contact with the two end-sections of the wire pattern. Particularly in one embodiment, the device 1000 includes a spacer region 71 and a spacer region 72 located between the gate region 60 and either the source region 31 or the drain region 32 along the wire pattern. The spacer region 71 is in contact with the insulation layer 20 including an undercut structure 21, and the spacer region 72 is in contact with the insulation layer 20 including an undercut structure 22. In another embodiment, the spacer regions 71 and 72 are in contact with the gate dielectric layer 50 at the two opposite ends of the cylindrical channel 30. In one example, the spacer region 71 or 72 may be made of insulation material including an atomic-layer-deposited an oxide-nitride-oxide (ONO) layer. It is to be understood that the insulation spacer regions may include other types of materials.

FIG. 4A is simplified diagram showing an device with a cylindrical semiconductor wire characterized by a source region, a drain region, and a gate surrounding the channel region (not shown) in the middle section, according to an embodiment of the present invention. FIG. 4B is simplified diagram of an x-y cross-section view of a gate/channel region showing an insulation layer disposed between the gate and the channel region and the formation of channel inversion layer (electron as the carrier). FIG. 4C is a simplified diagram showing an electric field distribution in the channel along the y-direction. These diagrams are merely examples for illustrating some device applications of the present invention, and should not unduly limit the scope of the claims. One of ordinary skill in the art would recognize many variations, alternatives, and modifications.

As an example, the device as illustrated according to FIGS. 4A, 4B, and 4C may have various characteristics: the channel region of the semiconductor wire may have an ideal cylinder shape with a length of L and a diameter of 2a; the gate may be a metal gate with a bias voltage V_(G) applied to it; the gate insulation layer may be a high-k oxide; the channel region may be a p-type silicon; the source region may be N⁺ implanted and may be grounded; and the drain region may be N⁺ implanted with a bias voltage V_(D) applied to it. The bias voltages V_(G) applied to the gate electrode helps create a channel inversion layer that is triggered at a certain threshold. A current flows from the source region across the channel region to the drain region. The electric field inside the channel can be modulated by the applied gate bias V_(G) to control the drain current.

With reference to FIGS. 4A-4C, the electrical properties of the semiconductor region, which in some cases can include band engineered semiconductor, can be described by Poisson's Equation, which relates the electrostatic potential and space charge distribution,

$\frac{\mathbb{d}^{2}\psi}{\mathbb{d}x^{2}} = {{- \frac{\mathbb{d}F}{\mathbb{d}x}} = {{- \frac{\rho_{s}}{ɛ_{s}}} = {{- \frac{e}{ɛ_{s}}}\left( {p - N_{A}} \right)}}}$ Within the P-type semiconductor depletion region w (completely depleted negative space charge region), this equation simplifies to the following,

$\frac{\mathbb{d}^{2}\psi}{\mathbb{d}x^{2}} = {{- \frac{\mathbb{d}F}{\mathbb{d}x}} = {{\frac{{eN}_{A}}{ɛ_{s}}\mspace{14mu}{for}\mspace{14mu} 0} \leq x \leq w}}$

The electric field distribution in the depletion region can be obtained by solving the Poisson's equation.

${{V(y)} + V_{G}} = {{V_{ox} + \psi_{s}} = {\frac{{eN}_{A}{wd}}{ɛ_{ox}} + \frac{{eN}_{A}w^{2}}{2ɛ_{s}}}}$ ${V_{Dsat} = {\frac{{eN}_{A}{ad}}{ɛ_{ox}} + \frac{{eN}_{A}a^{2}}{2ɛ_{s}} - V_{G}}},\mspace{14mu}\left( {{{when}\mspace{14mu} w} = a} \right)$ ${V_{G} = {\frac{{eN}_{A}w_{1}d}{ɛ_{ox}} + \frac{{eN}_{A}w_{1}^{2}}{2ɛ_{s}}}},\mspace{14mu}\left( {{{{at}\mspace{14mu} y} = 0},{{V(0)} = 0}} \right)$ ${{V_{G} + V_{D}} = {\frac{{eN}_{A}w_{2}d}{ɛ_{ox}} + \frac{{eN}_{A}w_{2}^{2}}{2ɛ_{s}}}},\mspace{14mu}\left( {{{{at}\mspace{14mu} y} = L},{{V(L)} = {VD}}} \right)$

The electric field distribution in a fully encapsulated cylindrical fec-FDFET device according to an embodiment of the present invention is symmetric in the cross section of the channel region surrounded by the gate of the transistor. The cross-sectional area for current flow is given by, A=π(a−w)² A=π(a−w)² where a is the radius of the cylinder and w is the depletion width at point of y. Thus, the resistance of the channel can be expressed by,

$R = {{\rho\frac{L}{A}} = {\frac{L}{e\;\mu_{p}N_{A}A} = \frac{L}{\pi\; e\;\mu_{p}{N_{A}\left( {a - w} \right)}^{2}}}}$

The current-voltage characteristic of the Fully Encapsulated Cylindrical fec-FDFET Device can be derived as follows.

${dV} = {{I_{D}{dR}} = \frac{I_{D}{dy}}{\pi\; e\;\mu_{p}{N_{A}\left( {a - w} \right)}^{2}}}$ I_(D)dy = π e μ_(p)N_(A)(a − w)²dV ${dV} = {\left( {\frac{{eN}_{A}d}{ɛ_{ox}} + {\frac{{eN}_{A}}{ɛ_{s}}w}} \right){dw}}$ ${I_{D}{dy}} = {\pi\; e\;\mu_{p}{N_{A}\left( {a - w} \right)}^{2}\left( {\frac{{eN}_{A}d}{ɛ_{ox}} + {\frac{{eN}_{A}}{ɛ_{s}}w}} \right){dw}}$ Integrating IDdy over the whole gate length (from y=0 to y=L), the transistor current can be expressed as:

$I_{D} = {{\frac{1}{L}{\int_{W_{1}}^{W_{2}}{\pi\; e\;\mu_{p}{N_{A}\left( {a - w} \right)}^{2}\left( {\frac{{eN}_{A}d}{ɛ_{ox}} + {\frac{{eN}_{A}}{ɛ_{s}}w}} \right)\ {\mathbb{d}w}}}} = {{\frac{1}{L}{\int_{W_{1}}^{W_{2}}{\pi\; e\;\mu_{p}{N_{A}\left\lbrack {\left( {a^{2} - {2\;{aw}} + w^{2}} \right)\left( {\frac{{eN}_{A}d}{ɛ_{ox}} + {\frac{{eN}_{A}}{ɛ_{s}}w}} \right)} \right\rbrack}\ {\mathbb{d}w}}}} = {{\frac{1}{L}{\int_{W_{1}}^{W_{2}}{\pi\; e\;\mu_{p}{N_{A}\left\lbrack {\frac{{eN}_{A}{da}^{2}}{ɛ_{ox}} + {\left( {\frac{{eN}_{A}a^{2}}{ɛ_{s}} - \frac{2\;{eN}_{A}{da}}{ɛ_{ox}}} \right)w} + {\left( {\frac{{eN}_{A}d}{ɛ_{ox}} - \frac{2\;{eN}_{A}a}{ɛ_{s}}} \right)w^{2}} + {\frac{{eN}_{A}}{ɛ_{s}}w^{3}}} \right\rbrack}{\mathbb{d}w}}}} = {\frac{1}{L}\pi\; e\;\mu_{p}{N_{A}\left\lbrack {{\frac{{eN}_{A}{da}^{2}}{ɛ_{ox}}w} + {\left( {\frac{{eN}_{A}a^{2}}{2ɛ_{s}} - \frac{{eN}_{A}{da}}{ɛ_{ox}}} \right)w^{2}} + {\left( {\frac{{eN}_{A}d}{3ɛ_{ox}} - \frac{2\;{eN}_{A}a}{3ɛ_{s}}} \right)w^{3}} + {\frac{{eN}_{A}}{4ɛ_{s}}w^{4}}} \right\rbrack}}}}}$

In an embodiment, the Fully Encapsulated Cylindrical fec-FDFET Device I-V Characteristics can be expressed as,

$I_{D} = {\frac{{{\pi\mu}_{p}\left( {eN}_{A} \right)}^{2}}{ɛ_{s}L}\left\lbrack {{\frac{ɛ_{s}}{ɛ_{ox}}{da}^{2}w_{2}} + {\left( {\frac{a^{2}}{2} - {\frac{ɛ_{s}}{ɛ_{ox}}{da}}} \right)w_{2}^{2}} + {\left( {\frac{ɛ_{s}d}{3ɛ_{ox}} - \frac{2\; a}{3}} \right)w_{2}^{3}} + {\frac{1}{4}w_{2}^{4}} - {\frac{ɛ_{s}}{ɛ_{ox}}{da}^{2}w_{1}} - {\left( {\frac{a^{2}}{2} - {\frac{ɛ_{s}}{ɛ_{ox}}{da}}} \right)w_{1}^{2}} - {\left( {\frac{d\; ɛ_{s}}{3ɛ_{ox}} - \frac{2\; a}{3}} \right)w_{1}^{3}} - {\frac{1}{4}w_{1}^{4}}} \right\rbrack}$   where $\mspace{20mu}{{w_{1} = {\sqrt{\left( {\frac{ɛ_{s}}{ɛ_{ox}}d} \right)^{2} + \frac{2ɛ_{s}V_{G}}{{eN}_{A}}} - {\frac{ɛ_{s}}{ɛ_{ox}}d}}},{and},\mspace{20mu}{w_{2} = {\sqrt{\left( {\frac{ɛ_{s}}{ɛ_{ox}}d} \right)^{2} + \frac{2{ɛ_{s}\left( {V_{G} + {VD}} \right)}}{{eN}_{A}}} - {\frac{ɛ_{s}}{ɛ_{ox}}d}}}}$ where

-   -   w1 and w2 represent the deplete width of the semiconductor at         the source and drain respectively;     -   a: silicon Cylinder radius;     -   L: gate length;     -   Co: gate oxide capacitance per area;     -   N_(A): Density of acceptor impurity atoms;     -   n_(i): intrinsic concentration of electrons;     -   μ_(n): electron mobility;     -   k: Boltzmann's constant;     -   e: electronic charge;     -   ∈_(s): dielectric constant; and     -   VD and VG are the voltages applied to the drain and the gate         respectively.

FIG. 5A is a simplified diagram showing drain current as a function of drain voltage for a gate-all-around cylindrical (GAAC) nanowire finFET at the different gate bias voltages according to an embodiment of the present invention. This diagram is merely an example, which should not unduly limit the scope of the claims. One of ordinary skill in the art would recognize many variations, alternatives, and modifications. For example, the GAAC nanowire finFET is the device 1000. As shown in FIG. 5A, a horizontal axis 401 represents the voltage applied to the gate region 60, and a vertical axis 402 represents the drain current flowing from source region 31 to the drain region 32, provided that the source region 31 is grounded. The channel region 30 has a cylindrical shape. For example, the channel region 30 has a cylinder radius of 10 nm and the channel length 35 is 10 nm, and the gate dielectric layer 50 is 1 nm in thickness. In a specific embodiment, the doped impurity density in channel is 5×10¹⁸ cm⁻³. The work function difference between the gate region 60 and the channel region 30 is −0.8V. For curves 410, 412, 414, and 416, the gate region is biased to 0.5V, 1V, 1.5V, and 2V, respectively. For example, the curves 410, 412, 414, and 416 each are obtained based on Eq. 1. As shown in FIG. 5A, the gate threshold voltage, at the device conditions provided above, is no larger than 0.5V.

FIG. 5B is a simplified diagram showing drain current as a function of gate voltage for a GAAC nanowire finFET with a drain voltage of 0.1V according to an embodiment of the present invention. This diagram is merely an example, which should not unduly limit the scope of the claims. One of ordinary skill in the art would recognize many variations, alternatives, and modifications. For example, device 1000 is a GAAC nanowire finFET. A horizontal axis 403 represents the voltage applied to the gate region 60, and a vertical axis 404, in logarithmic scale, represents the drain current flowing from the source region 31 to the drain region 32. For example, the graph is based on a scenario where the source region 31 is grounded and the drain region 32 is applied a bias voltage of V_(D)=0.1V. The cylindrical shaped channel region 30 has a cylinder radius of 10 nm and the channel length 35 is 10 nm. The gate dielectric layer 50 is 1 nm in thickness. The impurity density in the channel is 5×10¹⁸ cm⁻³. The work function difference between the gate region 60 and the channel region 30 is −0.8V. For curve 420, the drain region 32 is biased at 0.1V. For example, the curve 420 is obtained based on Eq. 1.

FIG. 5C is a simplified diagram showing drain current as a function of drain voltage for cylindrical type finFET devices with different gate separation widths. For zero gate separation width it becomes a GAAC nanowire finFET. For a non-zero gate separation width, it becomes a cylindrical Omega-gate type device. This diagram is merely an example, which should not unduly limit the scope of the claims. One of ordinary skill in the art would recognize many variations, alternatives, and modifications. For example, for the nanowire finFET with zero gate separation width is the device 1000.

As shown in FIG. 5C, a horizontal axis 405 represents the magnitude of the bias voltage applied to the gate region. A vertical axis 406 represents the current flowing from the source to drain. The gate separation width (SPA) is defined as the width of the insulation undercut structure under the channel region 30 of the wire pattern in the device 1000. It is understood that the term “SPA” is broadly defined and should not unduly limit the scope of claims. When SPA is zero, the ridge structure under the channel region 30 is removed so that the device is turned to be a GAAC nanowire finFET, (e.g., the device 1000). For curves 430, 431, and 432 the drain bias voltage of 0.8V is applied. For curves 433, 434, and 435 the drain voltage bias voltage of 0.05V is applied. For either cases with different drain bias, as the SPA is reduced from 50 Å to 10 Å and to 0, the drain current decreases at a fixed sub-threshold gate voltage. Among other things, the drain current decreases as a function of SPA decrease is an indication of a better performance in terms of off-current leakage for GAA type than others with a gate gap. This proves a clear advantage of the GAAC nanowire finFET device according to the present invention over the conventional multi-gate devices in terms of short-channel effect suppression.

FIG. 5D is a simplified diagram illustrating drain current as a function of gate voltage for a GAAC nanowire finFET with that of rectangular Tri-gate, Pi-gate, or Omega-gate device, respectively. This diagram is merely an example, which should not unduly limit the scope of the claims. One of ordinary skill in the art would recognize many variations, alternatives, and modifications. For example, the GAAC nanowire finFET is the device 1000.

As shown in FIG. 5D, a horizontal axis 407 represents the bias voltage applied to the gate region 60. A first vertical axis 408 represents the drain current on logarithmic scale. A second vertical axis 409 represent the same current in linear scale. Curves 441, 442, 443, and 444 are shown in the legend 490 and represent the results for rectangular Tri-gate device, Pi-gate device, Omega-gate device, and GAAC nanowire device under a fixed condition, respectively. The fixed condition is the same for all devices above, including a 0.5V bias applied to the drain region, a channel width of 4 nm (diameter of 4 nm in GAAC nanowire), a gate length of 9 nm, and other intrinsic physical properties of the material. The gate separation is 0.8 nm for Pi-gate device and 0.4 nm for Omega-gate device. In comparison, a GAAC nanowire device according to embodiments of the present invention has no gate separation. For example, the GAAC nanowire device is the device 1000. A first region 470 represents a condition that the device is under sub-threshold gate voltage. In region 470, the GAAC nanowire device shows the lowest leakage current as compared to other devices, with the sub-threshold swing value S of 70 mV/dec, 72 mV/dec, 80 mV/dec, and 84 mV/dec for curves 444, 443, 442, and 441, respectively, indicated within the bracket in the legend 490. A second region 480 of the graph represents a condition that the device is working at the above threshold. In region 480, the GAAC nanowire device shows the highest drain current with a same gate voltage applied and the same gate dimension, demonstrating a better performance of the GAAC nanowire device according to an embodiment of the present invention over the conventional multi-gate devices.

In some embodiments, the present invention further combines the advantages of improved electrical integrity in the gate and the simplicity of the fabrications for the 3-dimensional gate structure compatible with planar CMOS technology. The advantage to the electrical integrity in the gate includes, among other things, making the number of gates virtually infinite for the GAAC nanowire finFET device, consequently eliminating the corner-effect induced off-current leakage existed in the GAAR finFET device. In the cylindrical shaped channel surrounded by a gate, the electrical field is uniform in any cross-section along the channel. Because no corner exists for the GAAC nanowire finFET, there is no abrupt increase or accumulation of carrier charge in the channel, thereby reducing the potential off-current leakage and enhancing the capability to scale down the device. In addition, advantage to the simplicity of the fabrications for the GAAC nanowire finFET over GAAR finFET device lies in one or more simplified processes to form a nanowire bridge structure. After horizontally forming a silicon cylindrical nanowire on the insulator surface, an undercut structure (smaller than the width of the nanowire) made of the insulator is created, effectively reducing the structure size that is required to be removed by lateral etching. In contrast, in conventional GAAR finFET, the fin structure is rectangular shape, which makes it more difficult to form a cavity underneath with the lateral etching. An alternative type of GAA nanowire finFET device has an architecture including a vertical channel with a gate surrounding in horizontal plane. But the fabrication process for this type of GAA finFET is not quite compatible with the conventional planar CMOS technology to be cost effective.

According to one embodiment, the present invention provides a semiconductor device having a gate-all-around cylindrical nanowire architecture includes a substrate, a first insulation layer overlaying the substrate, and a semiconductor wire overlying the first insulation layer along a first direction. The semiconductor wire includes a first end section, a middle section, and a second end section. The semiconductor device further includes a source region within the first end section and a drain region within the second end section. Additionally, the semiconductor device includes a channel region within the middle section a channel region being within the middle section. The channel region connects the source region and the drain region. The channel region is characterized by a substantially cylindrical shape having a radius and a length. The device further includes a second insulation layer surrounding the cylindrical channel region. Moreover, the device includes a gate electrode overlaying the second insulation layer all around the channel region and overlaying the first insulation layer along a second direction which is substantially perpendicular to the first direction. For example, the device is implemented according to the device 1000.

According to another embodiment, the present invention provides a transistor with a gate surrounding a cylindrical nanowire channel includes a substrate, a first insulation layer overlaying the substrate, and a semiconductor wire overlying the first insulation layer along a first direction. The semiconductor wire includes a first end section, a middle section, and a second end section. The transistor further includes a source region within the first end section and a drain region within the second end section. Additionally, the transistor includes a channel region within the middle-section connecting the source region and the drain region. The channel region is characterized by a substantially cylindrical shape having a radius and a length. The transistor further includes a second insulation layer wrapped around the channel region. Moreover, the transistor includes a gate electrode surrounding the second insulation layer all around the channel region and overlaying the first insulation layer along a second direction. The second direction being substantially perpendicular to the first direction. The gate electrode is associated with a gate length along the first direction substantially equal to the length of the channel. Furthermore, the transistor includes a first spacer region and a second spacer region. The first spacer region is in contact with the gate electrode, the second insulation layer, the source region, and the first insulation layer. The second spacer region is in contact with the gate electrode, the second insulation layer, the drain region, and the first insulation layer. For example, the transistor is implemented according to the device 1000.

FIG. 6 is a simplified flow diagram illustrating a method for fabricating a semiconductor device having a gate-all-around cylindrical nanowire architecture according to an embodiment of the present invention. This diagram is merely an example, particularly using an SOI wafer substrate, which should not unduly limit the scope of the claimed herein. For example, various steps may be added, removed, replaced, repeated, overlapped, and/or partially overlapped. The method 2000 includes the following processes:

1. Process 2010 for preparing an SOI substrate;

2. Process 2020 for forming a wire pattern on the buried oxide layer along a first direction;

3. Process 2030 for defining a first end-section, a middle section, and a second end section along the wire pattern;

4. Process 2040 for forming cylindrical nanowire in the middle-section with a cavity underneath;

5. Process 2050 for forming gate dielectric layer at least around cylindrical nanowire;

6. Process 2060 for forming gate conductive layer;

7. Process 2070 for forming a gate region surrounding the cylindrical nanowire channel and overlaying the buried oxide layer in a second direction perpendicular to the first direction;

8. Process 2080 for forming insulation spacer regions and defining the source and drain regions within first and second end-sections, respectively;

9. Process 2090 for forming heavily doped source and drain regions;

10. Process 2100 for forming inner dielectric layer; and

11. Process 2110 for forming contacts to gate, source, and drain regions.

The above sequence of processes provides a method according to an embodiment of the present invention. Other alternatives can also be provided where processes are added, one or more processes are removed, or one or more processes are provided in a different sequence without departing from the scope of the claims herein. For example, the transistor with a gate-all-around cylindrical nanowire architecture made by the method 2000 is the device 1000. Future details of the present invention can be found throughout the present specification and more particularly below.

At the process 2010, an SOI wafer substrate including a bottom substrate, a buried oxide layer, and an SOI (i.e., silicon-on-insulator) layer, is provided. FIGS. 7A and 7B show a simplified method for preparing an SOI substrate for making device with a gate-all-around cylindrical nanowire architecture according to an embodiment of the present invention. These diagrams are merely examples, which should not unduly limit the scope of the claims. One of ordinary skill in the art would recognize many variations, alternatives, and modifications.

FIGS. 7A and 7B illustrate a perspective view and a side view (cutting along A-A′ plane) of a portion of an SOI wafer, respectively. The SOI wafer includes a bottom wafer substrate 100. For example, the wafer substrate 100 is a silicon wafer. On the substrate 100, a buried oxide layer 200 is overlaid. For example, the buried oxide layer 200 includes silicon oxide and is formed with a thickness about 100 nm to 300 nm by a thermal oxidation process. Additionally, an SOI layer 300 overlays the buried oxide layer 200. For example, the SOI layer 300 is made of a silicon material. In another example, the SOI layer 300 is a SiGe alloy or a multilayered substrate. In yet another example, the SOI layer 300 is thicker than 10 nm and thinner than 150 nm. In an embodiment, the SOI layer 300 is processed according to a threshold-adjustment ion implantation performed with n-type dopants, under a condition: Specie: B⁺, Dose: (1−5)×10¹² ions·cm⁻², Energy: (1−30)keV. In another example, a threshold-adjustment ion implantation for the SOI layer is performed with p-type dopants in the SOI layer 300, under the following condition: Specie: As⁺, Dose: (1−5)×10¹² ions·cm⁻², Energy: (1−20)keV. In yet another example, the final dopant concentration in the SOI layer 300 is around (1−10)×10¹⁸ cm⁻³.

At process 2020, the SOI layer and the buried oxide layer are patterned to form a wire pattern overlaying an undercut structure of the buried oxide layer in a certain direction. FIGS. 8A, 8B, and 8C show a simplified method for forming a wire pattern of the SOI layer on the buried oxide layer for making device with a gate-all-around cylindrical nanowire architecture according to an embodiment of the present invention. These diagrams are merely examples, which should not unduly limit the scope of the claims. One of ordinary skill in the art would recognize many variations, alternatives, and modifications.

As shown in FIGS. 8A, 8B and 8C, a wire pattern 301 is formed along an axis B-B′ overlaying an undercut structure 202 on the etched buried oxide layer 201. The wire pattern 301 includes silicon material from the SOI layer 300. The undercut structure 202 includes oxide material from the buried oxide layer 200. In one embodiment, the SOI layer 300 is patterned using photolithography mask to define a region along and around the axis B-B′. In another embodiment, the silicon active area etching is performed to partially etch away the SOI layer 300 and down into the buried oxide layer 200. In one example, the etching process involves a silicon plasma dry etch. The silicon active area etching yields the wire pattern 301 within the defined region along the axis B-B′, and a newly revealed surface of the etched oxide layer 201 outside of the defined region. In one embodiment, referring to FIG. 8B, the anisotropic wet etching is further performed at the oxide layer under the wire pattern 301 to produce the undercut structure 202. Nevertheless, the wire pattern 301 overlays the undercut structure 202 on the etched oxide layer 201. Namely, the width of the undercut structure 202 is less than the width of the wire pattern 301. In one example, the wire pattern 301 can be scaled down in size by utilizing thermal oxidation plus dilute HF solution dipping processes. FIG. 8B schematically shows the cross-section view cutting along the axis A-A′ of the wire pattern 301 overlaying the undercut structure 202 with a reduced width. FIG. 8C shows the cross-section view cutting along the axis B-B′ of the wire pattern 301 overlaying the undercut structure 202. The undercut structure 202 is a billet located above the dashed line and under the wire pattern 301.

At process 2030, a first end section, a middle section, and a second end section along the wire pattern formed at process 2020 are defined. FIGS. 9A, 9B, and 9C show a simplified method for defining first/second end sections and middle section of the wire pattern of the SOI layer on the buried oxide layer for making a device with a gate-all-around cylindrical nanowire architecture according to an embodiment of the present invention. These diagrams are merely examples, which should not unduly limit the scope of the claims. One of ordinary skill in the art would recognize many variations, alternatives, and modifications.

In one embodiment at the process 2030, photoresist layer is applied to cover the surface of the etched oxide layer 201 and the whole wire pattern 301 formed at process 2020 including the undercut structure 202. Lithography processing is performed with a pre-determined mask which exposes a middle section of the wire pattern 301 but covers two end sections. After the exposure, resist developing, and removal of the developed resist residue, the middle section of the wire pattern 301 with a lateral dimension L along B-B′ direction including a portion of undercut structure 202 beneath and portions of etched oxide layer 201 on both sides thereof are revealed. As shown in FIG. 9A, the unexposed resist patterns 351 and 352 remains to cover the two end sections of the wire pattern 301 including portions of the undercut structure 202 and the surface of etched oxide layer 201 on both sides thereof. Thus, three regions are defined along the wire pattern 301 at process 2030. A first end section 301 a is a portion of the wire pattern 301 covered by the resist pattern 351. Similarly, referring to FIG. 9A, a second end section 301 c is a portion of the wire pattern 301 covered by the resist pattern 352. The middle section 301 b, having a length L, is the portion of the wire pattern 301 revealed through lithography at process 2030. FIG. 9C shows a cross-section view along the B-B′ plane outlined in FIG. 9A, illustrating the defined sections 301 a, 301 b, and 301 c sequentially along the wire pattern 301.

In another embodiment, the undercut structure 202 underneath the first end section 301 a, the middle section 301 b, and the second end section 301 c is correspondingly divided by the process 2030 into three undercut portions 202 a, 202 b, and 202 c, respectively. FIG. 9B illustrates a cross-section view (along the A-A′ plane defined in FIG. 9A) of the first end section 301 a partially surrounded by the resist pattern 351 and undercut 202 a beneath. Specifically, the undercut portion 202 b possesses a same length L as the length of middle section 301 b of the wire pattern. Nevertheless at the end of process 2030, the undercut portions 202 a and 202 c are still covered by the resist layer 351 and 352 respectively, while undercut portion 202 b is revealed. Referring to FIG. 9A, the region not covered by the resist layer 351 or 352 determines a pattern masking that will be used for defining a gate region in subsequent processes.

At process 2040, a cylindrical shaped nanowire with a cavity underneath is formed. FIGS. 10A, 10B, and 10C show a simplified method for forming cylindrical nanowire with no contact to the buried oxide layer for making device with a gate-all-around cylindrical nanowire architecture according to an embodiment of the present invention. These diagrams are merely examples, which should not unduly limit the scope of the claims. One of ordinary skill in the art would recognize many variations, alternatives, and modifications. For example, FIGS. 10A, 10B, and 10C each illustrate a view of a partially processed integrated circuits.

At the end of process 2030, the middle section 301 b of the wire pattern and its undercut structure 202 b are revealed while end sections of the wire pattern and undercut structures are covered by the resist pattern 351 and 352. Under this condition, in one embodiment at process 2040 a lateral oxide etching process is performed using buffered oxide etchant (BOE) to selectively remove the revealed undercut portion 202 b with rest of the surface covered by etch-stop layer mask. Due to the narrowed width of the undercut structure created in earlier process 2020, this oxide etching process becomes greatly simplified. This process results in the formation of a cavity 205, as shown in FIG. 10C, and a complete exposure of the underneath portion of the middle section 301 b. In a preferred embodiment, the cavity has the same length L as the middle section 301 b.

At process 2040, silicon plasma etching followed by hydrogen annealing process at a temperature of 1000-1200° C. is performed to transform the middle section 301 b of the wire pattern into a substantially cylindrical shape by mass transportation around its periphery body. This hydrogen annealing process also eliminates damages to the wire body caused by the silicon etching process. In an embodiment, the combination of the etching and annealing process provides a well controlled cylindrical radius r. In yet another embodiment, the cylindrical body has a length substantially equal to the length L defined for the middle section 301 b. For example, the radius r is controlled to a range between 2 nm and 25 nm. In another example, the length L is controlled to within a range of 5 nm to 50 nm. Namely, a cylindrical nanowire with a controlled radius r and length L is formed at the middle section 301 b.

In one embodiment at the end of process 2030, referring to FIG. 10A, the resist patterns 351 and 352 are removed to obtain a device that includes wire pattern 301 having middle section 301 b being reshaped to a cylindrical nanowire in length L, cavity 205 underneath middle section 301 b, buried oxide layer 201 and two remained portions of undercut structures 202 a and 202 c. FIG. 10B also schematically illustrates a cross-section view of the cylindrical shaped nanowire 301 b with a radius r formed in the middle section of the wire pattern 301 and a gap between the nanowire and the etched oxide layer 201 due to the cavity 205.

At process 2050, a gate dielectric layer is formed covering the wire pattern at least around the cylindrical nanowire. FIGS. 11A, 11B, and 11C show a simplified method for forming gate dielectric layer at least around the cylindrical nanowire for making device with a gate-all-around cylindrical nanowire architecture according to an embodiment of the present invention. These diagrams are merely examples, which should not unduly limit the scope of the claims. One of ordinary skill in the art would recognize many variations, alternatives, and modifications.

As shown in FIG. 11A, according to an embodiment of the present invention, a gate dielectric layer 500 is deposited specifically to cover the wire pattern 301. Conventionally gate dielectrics is formed of an oxide layer or a nitride layer through a thermal oxidation or thermal nitrification process. In the described embodiment, the deposition of the gate dielectric layer 500 is performed by chemical vapor deposition technique with good step coverage. In another embodiment, atomic layer deposition technique is applied to the dielectric deposition for achieving finer uniformity and/or thickness control.

In one embodiment, the gate dielectric layer 500 at least is formed all around the peripheral cylindrical nanowire 301 b with the length L including its underneath surface in the cavity, as shown in FIG. 11B In another embodiment, the gate dielectric layer 500 has a thickness substantially less than the height of the cavity 205, leaving still a gap between the surface of the gate dielectric layer 500 and the etched oxide layer 201 (see FIG. 11C). For example, the thickness of the gate dielectric layer ranges from 1 nm to 3 nm. The material of the gate dielectric layer 500 is typically preferred to have a large bandgap and a high dielectric constant to provide excellent electrical insulation and to reduce the channel leakage current. In one example, the material of the gate dielectric layer 500 is metal oxide (e.g., aluminum oxide, tantalum oxide, titanium oxide, zirconium oxide, hafnium oxide, etc.).

At process 2060, the gate conductive layer is overlaid. FIGS. 12A, 12B, and 12C show a simplified method for overlaying gate conductive layer for making device with a gate-all-around cylindrical nanowire architecture according to an embodiment of the present invention. These diagrams are merely examples, which should not unduly limit the scope of the claims. One of ordinary skill in the art would recognize many variations, alternatives, and modifications.

As shown in FIGS. 12A, 12B, and 12C, a conductive material layer 600 covers the wire pattern 301 (after the coating of the gate dielectric layer 500) and the exposed surface of etched oxide layer 201 including the undercut structures 202 a and 202 c and the cavity 205 underneath the surrounding gate dielectric layer 500. Prior to the deposition of the conductive gate material 600, an etch-stop layer pattern is applied with the same masking pattern as in process 2030 for defining the cylindrical nanowire 301 b of wire pattern 301 and the region beyond the both sides thereof. This step defines a gate region which is co-centered with the cylindrical nanowire 301 b with a lateral dimension of L along the direction of B-B′ and extends beyond the both sides of the middle-section 301 b in the direction A-A′. Preferably, according to an embodiment of the present invention, a conductive gate material is deposited using CVD technique. In one example embodiment, a low pressure (LP) CVD is applied, which ensures the stacked conductive layer 600 filling any cavity and being able to entirely surround the cylindrical nanowire 301 b with the thin gate dielectric layer 500 interposed inbetween. In another example embodiment, the conductive gate material is a poly-silicon layer being grown at a temperature range of 600° C. to 800° C. and having a thickness ranging from 100 nm to 500 nm and being heavily doped with an n-type impurity. In yet another example embodiment, the conductive gate material is a metal layer grown by a LPCVD process and has a thickness in a range between 10 nm and 200 nm.

Now referring back to FIG. 6. At process 2070, a gate electrode is formed. FIGS. 13A, 13B, and 13C show a simplified method for forming a gate electrode surrounding the cylindrical nanowire channel and overlaying the buried oxide layer in a second direction perpendicular to the first direction for making a device with a gate-all-around cylindrical nanowire architecture according to an embodiment of the present invention. These diagrams are merely examples, which should not unduly limit the scope of the claims. One of ordinary skill in the art would recognize many variations, alternatives, and modifications. For example, the process is implemented in manufacturing the device 1000.

At process 2070, a resist layer is formed and patterned on the same gate region defined at the process 2060. Then a conductive layer etching is performed to remove portions of the gate material that are not covered by this patterned resist layer. The etching is stopped at the etch-stop layer placed in the process 2060 over the first end-section 301 a and the second end-section 301 c of the wire pattern 301. The resist layer is then stripped through phosphoric wet-etching. As a result, according to one embodiment of the present invention, as shown in FIGS. 13A, 13B, and 13C, a gate electrode 610 made of the conductive material 600 is formed at the lithography-defined gate region. The gate electrode 610 surrounds the cylindrical nanowire 301 b with an interposed gate dielectric layer 500. In one embodiment, as shown in FIG. 13A, the gate electrode 610 is associated with a gate length of L along the wire pattern that is substantially equal to the length of the cylindrical nanowire 301 b. In another embodiment, the gate electrode 610 is expanded along A-A′ direction overlaying the etched oxide layer 201 while the wire pattern 301 vertically penetrates through thereof along B-B′ direction in a state so that the gate dielectric layer 500 is interposed. In yet another embodiment, referring to FIGS. 13B and 13C, the formation of the gate electrode 610 naturally defines the channel region 330 for the device. The channel region 330 is the cylindrical nanowire formed at process 2040 which is surrounded first by the gate dielectric layer 500 then by the gate electrode 610. In one example, the cylindrical nanowire channel 330 is made of silicon. The silicon nanowire pattern is formed from the SOI layer doped through the threshold-adjustment ion implantation.

Referring back to FIG. 6, at process 2080, spacer regions between the gate electrode and the wire pattern are formed, thereby defining the source region and the drain region. FIGS. 14A, 14B, and 14C show a simplified method for formingspacer regions and defining the source and drain regions for making device with a gate-all-around cylindrical nanowire architecture according to an embodiment of the present invention. These diagrams are merely examples, which should not unduly limit the scope of the claims. One of ordinary skill in the art would recognize many variations, alternatives, and modifications.

At the end of the process 2070, as shown in FIG. 13A, the formation of the gate electrode 610 surrounded the channel region 330 reveals the two opposite end-sections 301 a and 301 c of the wire pattern 301. As a result, two circular intersection lines (only one is viewable in FIG. 13A) between the wire pattern 301 and the gate electrode 610. Along the peripheral intersection line, at least four materials or layers including the silicon in wire pattern 301, gate dielectric layer 500, etched oxide layer 201, and gate conductive layer 600 are partially in contact with each other.

Referring to FIG. 14A, in one embodiment at the process 2080, spacer regions 710 and 720 are formed to cover the areas around the two intersection lines. In another embodiment, the formation of the spacer regions 710 and 720 serves as an electrical isolation and functions as a diffusion barrier between the conductive gate electrode and the wire pattern 301. In another embodiment, the spacer regions 710 and 720 also physically define the wire pattern section 301 a and 301 c beyond the two spacer regions 710 and 720 as the source region 310 and drain region 320, respectively.

As shown in FIGS. 14A and 14C, the spacer region 710 is in contact with the gate electrode 610, the source region 310, the gate dielectric layer 500 and the buried oxide layer 201 including part of the undercut structure 202 a, the spacer region 720 is in contact with the gate electrode 610, the drain region 320, the gate dielectric layer 500 and the buried oxide layer 201 including part of the undercut structure 202 c. In another embodiment, the spacer regions 710 and 720 are made of a dielectric material. In one example, it is an oxide-nitride-oxide (ONO) layer. Particularly, the ONO layer can be a sequential layer of silicon oxide, silicon nitride, and silicon oxide. In another example, the deposition of the spacer layer is performed through CVD, PVD, or ALD techniques. A patterned resist mask layer is usually to be applied prior to the spacer layer deposition and subsequently removed after the deposition.

At process 2090, the heavily doped source/drain regions are formed. FIGS. 14A, 14B, and 14C can also be used to illustrate a simplified method for forming the source and drain regions by heavily implanting the dopants with opposite polarity to the channel dopant for making a device with a gate-all-around cylindrical nanowire architecture according to an embodiment of the present invention. Referring to FIGS. 14A and 14C, according to an embodiment of the present invention, the source region 310 is formed by heavily doping the wire pattern section 301 a in the portion defined at process 2080. In the same described embodiment, the drain region 320 is formed by heavily doping the wire pattern section 301 c in the portion defined at process 2080. In another embodiment, the dopant polarity of source and drain regions is opposite to the channel doping polarity which is set during the initial SOI wafer preparation at process 2010. For example, for an N-type finFET device the channel region 330 needs to be doped to n-type, thereby the source region 310 and the drain region 320 need to be heavily doped with p-type dopants by ion implantation. In another example, for a P-type finFET device the channel region 330 needs to be doped to p-type, thereby the source region 310 and the drain region 320 need to be heavily doped with n-type dopants by ion implantation. For each ion implantation process, relevant implantation mask is applied to corresponding areas/regions before the performance of implantation towards the source/drain region specifically. In one example, the N⁺ S/D implant condition is: Specie As⁺, energy 60 KeV, dosage 5.5×10¹³ ions/cm², tilt angle of 0 degree. In another example, the P⁺ S/D implant condition is: Specie B⁺, energy 5 KeV, dosage 3.5×10¹³ ions/cm², tilt angle of 0 degree.

Now referring back to FIG. 6. At process 2100, an inner dielectric layer is formed. FIGS. 15A and 15B show a simplified method for forming inner dielectric layer for making device with a gate-all-around cylindrical nanowire architecture according to an embodiment of the present invention. These diagrams are merely examples, which should not unduly limit the scope of the claims. One of ordinary skill in the art would recognize many variations, alternatives, and modifications.

Referring to FIGS. 15A and 15B, according to an embodiment of the present invention, an inner dielectric layer 800 is formed to cover the gate electrode 610, the source region 310 and the drain region 320, spacer regions 710 and 720, and partially the etched oxide layer 201. In one example, the inner dielectric layer 800 is a silicon oxynitride layer being grown in a rapid thermal processor with NH₃, N₂O, or NO gas flow therein, followed by a BPSG layer with reflow process, then overlaid with an oxide layer planarized by a CMP process.

Now referring back to FIG. 6. At process 2110, one or more contacts are formed. FIGS. 15A and 15B also show a simplified method for forming contacts to the gate, source, and drain regions for making device with a gate-all-around cylindrical nanowire transistor according to an embodiment of the present invention. These diagrams are merely examples, which should not unduly limit the scope of the claims. One of ordinary skill in the art would recognize many variations, alternatives, and modifications.

As shown in FIGS. 15A and 15B, in one example, contact holes 910 and 920 is formed to expose the doped silicon wire pattern on the source region 310 and the drain region 320 respectively. In another example, a contact hole 930 similarly exposes the gate electrode 610 at a position not in the B-B′ plane (not seen in FIG. 15B). In the contact holes, a diffusion barrier layer is formed to cover the bottom and the side surfaces of the holes 910, 920 and 930. For example, the diffusion barrier layer includes titanium (Ti) and Titanium Nitride (TiN) material. After contact holes are formed, the contact holes 910, 920, and 930 are filled with a metal material including tungsten, forming the contacts to the source region 310, the drain region 320, and the gate electrode 610, respectively

According to one embodiment, a method for fabricating a semiconductor device having a gate-all-around cylindrical nanowire architecture is provided. The method includes providing a first insulation layer on a wafer substrate and forming a semiconductor wire overlying the first insulation layer along a first direction. The semiconductor wire includes a first end section, a middle section, and a second end section. The method further includes forming an undercut structure of the first insulation layer beneath the semiconductor wire and selectively removing the undercut structure beneath the middle section to form a cavity having a first length and a first height. Additionally, the method includes forming a channel region by shaping the middle section above the cavity to a substantially cylindrical shape. The channel region is associated with a channel length substantially equal to the first length. The method further includes forming a second insulation layer overlying around the cylindrical channel region. The second insulation layer has a thickness substantially less than the first height of the cavity. Moreover, the method includes depositing a conductive layer on the first insulation layer to at least cover the semiconductor wire including the cylindrical channel region surrounded by the second insulation layer and to fill the cavity beneath. The method includes further forming a gate region from the conductive layer along a second direction surrounding the cylindrical channel region including the filled cavity beneath. The second direction is substantially perpendicular to the first direction. The gate region is characterized by a gate length along the first direction which is substantially equal to the first length. Furthermore, the method includes forming a source region in the first end section and forming a drain region in the second end section. For example, the method is illustrated by the method 2000.

According to another embodiment, a method of making a transistor with a gate surrounding a cylindrical nanowire channel includes providing an SOI substrate including an SOI layer, a buried oxide layer and a bottom substrate, and forming a wire pattern within the SOI layer on the buried oxide layer along a first direction. The wire pattern includes a first end section, a middle section, and a second end section. The method further includes forming an undercut structure of the buried oxide layer underneath the wire pattern and selectively removing the undercut structure underneath the middle-section to form a cavity having a first length and a first height. Additionally, the method includes forming a channel region by shaping the middle section above the cavity to a substantially cylindrical shape. The channel region is associated with a channel length substantially equal to the first length. The method further includes forming a gate dielectric layer at least around the cylindrical channel region. The gate dielectric layer has a thickness substantially less than the first height of the cavity. The method further includes depositing a conductive layer on the buried oxide layer to at least cover the wire pattern including the channel region surrounded by the gate dielectric layer and to fill the cavity underneath. Moreover, the method includes forming a gate region from the conductive layer along a second direction surrounding the cylindrical channel region including the filled cavity underneath. The second direction is substantially perpendicular to the first direction. The gate region is associated with a gate length along the first direction which is substantially equal to the first length Furthermore, the method includes forming a source region in the first end-section and a drain region in the second end-section. The method further includes forming a first spacer region and a second spacer region. The first spacer region is in contact with the gate region, the source region, the gate dielectric layer, and the buried oxide layer; and the second spacer region is in contact with the gate region, the drain region, the gate dielectric layer, and the buried oxide layer. For example, the method is implemented according to method 2000.

In still another embodiment, a method for forming a semiconductor devices includes providing a semiconductor layer overlying an insulator layer and forming an elongated semiconductor region in the semiconductor layer using an anisotropic semiconductor etching process. The method includes etching the insulating layer using an isotropic dielectric etching process, forming a mask to protect a first portion and a second portion of the elongated semiconductor region and expose a third portion of the elongated semiconductor region, and removing a portion of the insulator layer under the third portion of the elongated semiconductor region to form a void region. The method also includes forming a dielectric layer overlying the elongated semiconductor region and forming a conductive material overlying the dielectric layer over the elongated semiconductor region, the conductive material completely surrounding the third portion of the elongated semiconductor region. The method further includes patterning the conductive material.

In some embodiments, the method also includes using an isotropic dielectric etch process to form undercuts under the elongated semiconductor region. In another embodiment, the semiconductor layer overlying the insulator layer is provided on a silicon-on-insulator (SOI) substrate. In yet another embodiment, the method also includes thermal oxidation of the elongated semiconductor region and isotropic oxide etch to cause the elongated semiconductor region to have a rounded cross-section. In another embodiments, an isotropic dielectric etch is used for removing a portion of the insulator layer under the third portion of the semiconductor region. In a specific embodiment, a buffered oxide etchant (BOE) is used for removing a portion of the insulator layer under the third portion of the semiconductor region.

In some embodiments, the method also includes an H₂ annealing process for forming a cylindrical-shaped semiconductor region. In an embodiment, the H₂ annealing process is carried out at a temperature range of about 1000° C. to about 1200° C. In another embodiment, the cylindrical-shaped semiconductor region has a diameter of from about 2 nm to about 25 nm. In another embodiment, forming the dielectric layer comprises forming a high-K dielectric layer. In an embodiment, forming the conductive material includes forming a polysilicon material. In another embodiment, the method also includes forming ONO (Oxide/Nitride/Oxide) spacers adjacent to the patterned conductive material.

Embodiments of the present invention have many advantages. Some embodiments of the present invention provide a device having a gate architecture that entirely surrounds a cylindrical shaped channel region having a thin high-k gate dielectric layer interposed. For example, the channel region of the device is characterized by a nano-scale cylindrical silicon wire forming a bridge structure between the source region and the drain region along the same wire pattern, and the gate region is formed substantially perpendicular to the wire pattern on a buried oxide layer of an SOI substrate. The device can be controlled by an infinite number of gates surrounding the cylinder-shaped channel. The device having such a gate architecture possesses a much improved electrical integrity. The current leakage due to the corner effect of the conventional gate-all-around fin transistor is substantially reduced. The present invention demonstrates that the gate-all-around cylindrical nanowire FinFET device provides superior scaling capability with suppressed short-channel effects and more stable gate control than the conventional multi-gate finFET device. Additionally, the present invention provides a fabrication process of the gate-all-around cylindrical nanowire finFET device characterized by its simplicity and full compatibility with conventional planar CMOS technology.

Additional embodiments of the present invention will be described in detail herein below with references to the drawings.

FIG. 16 is a simplified perspective view of a conventional finFET device. The finFET device of FIG. 16 includes a semiconductor substrate 15 and a buried oxide layer 16 on the semiconductor substrate. The finFET includes a rectangular fin structure 17 and a gate structure overlying a top portion and side portions of the fin structure. The gate structure comprises a gate dielectric layer (not shown) and a gate electrode 18. The top portion and the side portions of the fin structure surrounded by the gate structure form the channel region of the finFET device. The finFET device thus includes multiple gate regions and has increased drain current and improved performance.

Such finFET device structure is discussed in U.S. Publication No. 2011069101 A1 to Doornbos et al. The discussed finFET device includes a semiconductor substrate and a fin structure that has a top portion and a bottom portion. The top portion of the fin structure is doped with a first conductivity type dopants and the bottom portion is doped with a second conductivity type dopants. The top and bottom portions form a junction.

However, as the device geometry decreases, the space between regions of different dopants is getting smaller, and diffusion will occur between the different dopants that may affect the finFET device stability and increase the difficulty in manufacturing the finFET device.

Embodiments of the present invention provide a transistor device where the source region, the drain region, and the channel region are doped with dopants of a same polarity and a same doping concentration. Thus, the transistor device, in accordance with the present invention, does not have a junction between source and channel regions or between drain and channel regions. In other words, the transistor is junction-less. The transistor device further includes a fin structure protruding over the semiconductor substrate. The fin structure includes a bottom portion that has a rectangular cross-section and a top portion that has a semi-circular cross-section so that the fin structure has a cross sectional profile of a reversed U shape or a semicircular window shape. In accordance with the present invention, the source and drain regions are disposed on opposite ends of the fin structure, and a gate structure is overlying the channel region that is disposed between the source and drain regions.

Embodiments of the present invention also provide a method for fabricating a transistor device. The method includes providing a semiconductor substrate, forming an elongated semiconductor structure overlying a region of the semiconductor substrate along a first (e.g., horizontally relative to the surface of the substrate) direction, and etching the elongated semiconductor structure to form an arched top portion so that the elongated semiconductor structure has a cross section perpendicular to the first direction that is in the form of a reversed U or a semi-circular window. The method further includes implanting dopants of a first type impurity in the elongated semiconductor structure and depositing a conductive layer on the elongated semiconductor structure.

FIGS. 17 through 19 each show a simplified diagram of a transistor device 1700 having a cross-section of semi-circular window shape according to an embodiment of the present invention. These diagrams are merely examples, which should not unduly limit the scope of the claims. One of the ordinary skills in the art should recognize many variations, alternatives, and modifications. FIG. 17 shows a perspective view of the device 1700. FIG. 18 illustrates a cross-section view cutting along an A-A′ plane of the FIG. 17. The A-A′ plane as shown is a vertical plane along the axis A-A′ in FIG. 17. FIG. 19 illustrates a cross-section view cutting along a B-B′ plane of the FIG. 17. The B-B′ plane is a vertical plane along the axis B-B′ in FIG. 2. The device 1700 includes a substrate region 100, an insulation layer 101, a fin structure 103, a gate dielectric layer 104, and a gate electrode 102. In one embodiment, the device may include spacers 105 disposed adjacent to side walls of gate electrode 102 and gate dielectric layer 104. In another embodiment, insulation layer 101 may be buried oxide layer of an SOI wafer substrate, fin structure 103 may include a semiconductor material, gate dielectric layer 104 may be silicon oxide or silicon nitride, and gate electrode 102 may include a metallic material or a polysilicon material.

Device 1700 includes a source region “S” and a drain region “D” disposed in the opposite end-sections of fin structure 103, and a channel region “C’ surrounded by gate electrode 102 and disposed between the source and drain regions. In an embodiment, the source, drain, and channel regions each are doped with dopants of the same polarity and have the same doping concentration so that the device is a junction-less transistor. In an embodiment, the source, drain, and channel regions each are doped heavily with an n-type impurity for an NMOS transistor device. In another embodiment, the source, drain, and channel regions each are doped heavily with a p-type impurity for a PMOS transistor device. In an embodiment, the device may include a control voltage that is applied to gate electrode 102 to control the current flow between the source and drain regions.

Referring still to FIGS. 17 through 19, the cross-sectional dimension of the fin structure along the A-A′ plane is the surface area available for the current flow between the source and drain regions. Therefore, a large cross-sectional area of the fin structure along the A-A′ plane reduces the resistance between the source and drain regions. In an embodiment, fin structure 203 includes a rectangular cross-section bottom portion and an arched cross-section top portion. In one embodiment, the arched cross-section top portion is a half circle having a radius “r” with the flat portion of the half circle abutted to the top surface of the rectangular cross-section bottom portion. The rectangular cross-section bottom portion has thus a width equal to 2r and a height “h”. In a preferred embodiment, the height h is greater than or equal to the radius r to obtain a large cross-sectional area of the fin structure along the A-A′ plane.

It is noted that the doping concentration should be high to obtain a complete ON state or a complete OFF state of the channel. In other words, the channel is completely depleted of carriers when in the OFF state. In an embodiment, the fin structure has a doping concentration that is approximately 2×10¹⁹ atoms/cm³ or greater.

FIGS. 20 through 23 show an exemplary operation of a PMOS transistor device according to the present invention. As shown in FIG. 20, the source, drain, and channel regions are heavily doped (i.e., ≧2×10¹⁹ atoms/cm³) with a P-type impurity. A voltage of 0 V is applied to gate electrode 102 and the source region, a negative voltage is applied to the drain region. Holes in the channel region cause a current flowing between the source and drain regions; the PMOS transistor device is in the ON state. In an embodiment, the voltage applied to the gate electrode can be negative so that holes are then accumulated in the channel region to increase the current flow between the source and drain regions.

When a small amount of positive voltage is applied to gate electrode 102, holes in the channel region are repelled from the channel region. In other words, the channel region is partially depleted of holes, a depletion region between the source and drain regions is formed as shown in FIG. 21.

When the amount of positive voltage applied to gate electrode 102 keeps increasing, holes in the channel regions are then completely repelled from the channel region as shown in FIG. 22, and the PMOS transistor device is in an OFF state, that is, the resistance value between the source and drain regions becomes very large, or infinite in an ideal condition.

It will be appreciated that the above described exemplary operation can be similarly applied to an NMOS transistor device according to the present invention. For an NMOS transistor device, the source, drain, and channel regions are heavily doped (i.e., ≧2×10¹⁹ atoms/cm³) with an n-type impurity. The drain region has a voltage potential higher than that of the source region. When the voltage applied at the gate electrode is zero or below a threshold voltage, the channel region is depleted of electrons (full depletion) so that the resistance between the source and drain regions is very high (e.g., infinite in the ideal case). The NMOS transistor device is in an OFF state. When the gate voltage is above the threshold voltage, electrons are accumulated in the channel regions (partial depletion), and some current will flow between the drain and source regions. When the gate voltage increases to a value much higher than the threshold voltage, the channel region is no longer depleted of electrons and becomes fully conducting. The NMOS transistor device is in the ON state.

FIGS. 23 through 25 show another embodiment of the present invention. FIG. 23 shows a simplified perspective view of a transistor device 2000. FIG. 24 is a simplified cross-sectional view of transistor device 2000 along the C-C′ plane, and FIG. 25 is a simplified cross-sectional view of transistor device 2000 along the D-D′ plane of FIG. 23. The C-C′ plane as shown is a vertical plane along the axis C-C′ of FIG. 23.

Device 2000 includes a silicon substrate 200, one or more fin structures 203 (only one is shown for the sake of clarity) disposed on the silicon substrate, and an insulation layer 201 having a plurality of insulating regions disposed in the silicon substrate and configured to insulate the fin structures from each other. The insulating regions 201 may include shallow trench isolation structures having a silicon oxide material. In an embodiment, each of the fin structures is disposed on a region of the silicon substrate that is surrounded by the insulation layer. Each of the fin structures may include a rectangular cross-section bottom portion and a semi-circular cross-section top portion that is abutted on the top surface of the bottom portion. For reason of brevity, only one fin structure will be described in detail below.

As shown in FIG. 23, fin structure 203 extends in a horizontal direction along the axis D-D′ and have a source region and a drain region disposed at both distal ends. A gate insulating layer 204 is disposed on a region of the fin structure between the source and drain regions, and a gate electrode 102 is disposed on the gate insulating layer. A channel region is thus formed between the source and drain regions and has a length that is about the length L of gate electrode 102. The gate electrode has a width that spans over the fin structure from the insulation region on one side of the fin structure to the other side of the fin structure in the A-A′ direction, which is perpendicular to the B-B′ direction. In an embodiment, gate insulating layer 104 may comprise a silicon oxide or a silicon nitride material. The gate electrode may include a metallic material or a polysilicon material.

In an embodiment, the fin structure may be doped with a first-type impurity. When the device is a PMOS transistor, the first-type impurity is a p-type impurity. In another embodiment, when the device is an NMOS transistor, the first-type impurity is an n-type impurity.

In the following exemplary embodiment, the device is a PMOS transistor so that the fin structure is heavily doped with p-type dopants, i.e., the source, drain, and the channel regions have the same dopant polarity and substantially the same doping concentration. In a preferred embodiment, the doping concentration may be 2×10¹⁹ atoms/cm³ or greater. The substrate region in contact with the bottom portion of the fin structure is doped with n-type dopants. The following section describes an example operation of a PMOS transistor that is similar to the operation of the device 1700 described in previous sections. When a ground voltage, i.e., zero volt, is applied to the gate electrode, carriers (holes) are accumulated in the channel region below the gate electrode, and a current flows between the source and drain regions. The device is in the ON state. With the increase of the gate voltage, carriers are repelled from the channel region and the depletion region between the source and drain regions expands until the carriers are fully depleted, and the current flow between the source and drain regions stops.

In another embodiment, the fin structure may be doped with n-type dopants. When a voltage applied to the gate electrode is below a threshold voltage, the channel region is depleted of electrons (full depletion), and the device is in the OFF state. With the increase in the gate voltage, for example the gate voltage is higher than the threshold voltage, some electrons are accumulated in the channel region (partial depletion) and cause a current to flow between the drain and source regions. With the gate voltage much higher than the threshold voltage, more electrons are in the channel regions and lower the resistance of the channel region, and the transistor device is in an ON state.

In an embodiment, the semi-circular fin top portion with a radius “r” is abutted to the top surface of the rectangular cross-section bottom portion that has a height “h” equal to or greater than the radius r to obtain a large cross-sectional interface area in the channel region between the source and drain region. A large cross-sectional area of the channel region provides improved device characteristics of the transistor device as described in above sections.

FIG. 26 is a simplified flow diagram illustrating a method 2600 for fabricating one or more transistor devices having a fin structure that is semi-circular window shaped, in accordance with an embodiment of the present invention. This diagram is merely an example, which should not unduly limit the scope of the claims. One of ordinary skill in the art would recognize many variations, alternatives, and modifications.

Referring to FIG. 26, a method 2600 for fabricating a transistor device according to embodiments of the present invention can be outlined as follows:

-   -   1. Provide a semiconductor substrate (process 2610);     -   2. Form an elongate semiconductor structure on a region of the         semiconductor substrate (process 2620);     -   3. Etch the elongate semiconductor structure to obtain an arched         top portion (process 2630);     -   4. Implant dopants of a first-type in the elongate semiconductor         structure (process 2640);     -   5. Form a gate structure on a portion of the elongate         semiconductor structure (process 2650);     -   6. Form spacers on the gate structure (process 2660); and     -   7. Form contacts and interconnects to gate, source, and drain         regions (process 2670).

The sequence of processes is further illustrated in FIGS. 27 through 30. Referring to FIG. 27 and process step 2610, a semiconductor substrate including a silicon layer 300 and a silicon oxide layer 301 on the silicon layer is provided. In process 2620, an elongated semiconductor structure 303 is provided on the silicon oxide layer. In an embodiment, elongated semiconductor structure 303 may comprise silicon, germanium, silicon germanium, or other III or V semiconductor materials. The formation of an elongated semiconductor structure may use conventional techniques that are well known and will not be described herein for the reason of brevity.

In process 2630, an etch process is performed at least once on the elongated semiconductor structure. The etch process may include thermal oxidization to obtain an oxide layer. As shown in FIG. 27, convex corner regions (indicated as “α”) each have an angle of 270 degrees and side surfaces (indicated as “β”) each have an angle of 180 degrees. Thus, the convex corner regions have a larger surface exposed to oxygen than the side surfaces. As a result, the oxide layer formed around the corner regions is thicker than the oxide layer formed on the side surfaces. An etch process with a high silicon oxide etch selectivity over silicon can be used, such as a diluted HF wet etching process. Because the oxide layer around the corner regions has a larger thickness than the side surfaces, the etch process will remove the oxide layer around the corner regions more than the oxide layer on the side surfaces. In an embodiment, the thermal oxidization and the wet etching process can be repeated multiple times so that the corner regions will take on an arched profile. In an embodiment, a hydrogen annealing process may be performed after etching to obtain a cross-sectional semi-circular shape from the arched top surface of the elongated semiconductor structure, as shown in FIG. 28.

Therefore, the present invention provides a method of forming an arched top portion of the elongated semiconductor structure using conventional manufacturing techniques. In an embodiment, the elongated semiconductor structure has a semi-circular cross-section top portion abutted to a rectangular cross-section bottom portion to form a reversed U-shaped profile or a semi-circular window-shaped profile. In order to obtain a large cross-sectional area of the semi-circular window-shaped profile, the height of the rectangular bottom portion is equal to or greater than the radius of the semi-circular top portion in a preferred embodiment. As discussed above, a large cross sectional area provides better performance to the device.

In an embodiment, the hydrogen annealing process is performed at a temperature range from about 800° C. to 1200° C. and for a time period ranging from 5 minutes to 30 minutes. The annealing process allows the surface of the elongated semiconductor structure to be smooth and the arched top portion of the elongated structure to have a semi-circular profile. The smooth surface prevents carriers from scattering, enhances the movement of carriers in the channel region, and thus improves the performance of the device.

The method further includes implanting dopants of a first-type impurity in the elongated semiconductor structure, as shown in FIG. 29 together with process 2640. In an exemplary embodiment, the first type impurity is an n-type dopant for fabricating an NMOS transistor device. The n-type dopant may include phosphorous (P), arsenic (As), and/or antimony (Sb). In another exemplary embodiment, the first type impurity is a p-type dopant for fabricating a PMOS transistor device, as shown in FIG. 29. The p-type dopant may include boron (B), indium (In), gallium Ga), and a combination thereof. In an embodiment, the doping concentration is about 2×10¹⁹ atoms/cm³ or greater to obtain a full depletion region.

The method further includes forming a gate structure on the elongated semiconductor structure in process 2650. As shown in FIG. 30, the gate structure includes a gate dielectric layer 304 and a gate electrode 302 on the gate dielectric layer. In an embodiment, gate dielectric layer 304 may include a silicon oxide that is formed by thermal oxidation at a temperature range between 750 degrees and 800 degrees Celsius. In another embodiment, gate dielectric layer 304 may be formed by deposition of an insulating material, such as nitride or a high dielectric-constant material. Gate electrode 302 may include any suitable electrically conductive material, such as polysilicon, amorphous silicon, polycrystalline silicon, silicon germanium, W, Mo, Ta, other refractory metal, Ni, Co, or a combination thereof. The gate electrode can be formed by physical vapor deposition or chemical vapor deposition Polysilicon.

Lithography processing can be performed with a patterned mask that covers the two opposite end sections and exposes the section disposed therebetween to sequentially form dielectric layer 304 and gate electrode 302. The photolithography process may include forming a gate patterned photoresist as a mask for etching a hard mask layer, and then etching the gate material layer and the gate dielectric layer using the hard mask layer. The etching can preferably be a plasma dry etching for good anisotropy in an embodiment.

At process 2660, spacers (not shown) may be formed on the sidewalls of gate dielectric layer 304 and gate electrode 302 to protect the gate dielectric layer and the gate electrode from damage in subsequent processes. Spacers are made of a dielectric material. In one embodiment, the spacers may include an oxide-nitride-oxide (ONO) layer. The spacers may be formed using CVD, PVD, or ALD techniques.

In an embodiment, fin structure 303 is doped with a P-type dopant impurity, each exposed end portions of the fin structure forms a source region or a drain region, and the gate structure comprising gate dielectric layer 304 and gate electrode layer 302 is overlying the channel region between the source and drain regions. At process 2670, an interlayer dielectric layer (not shown) is former overlying the gate electrode, the source and drain regions, and the spacers. The interlayer dielectric layer may be a thermally grown silicon oxynitride layer. Contact holes are formed, diffusion barrier layer are then formed to cover the bottom and side surfaces of the contact holes, and filled thereafter with a metallic material to form contacts to the source and drain regions and to the gate electrode. These processes have been described in detail in sections above and won't be repeated herein for the sake of brevity.

FIGS. 31 through 34 illustrate a process for fabricating an arched-top finFET transistor device in accordance with one exemplary embodiment of the present invention. As shown in FIG. 31, a plurality of trenches 405 is formed in a silicon substrate 400. The trenches are separated from each other so that a protruding elongated silicon structure 403 is disposed between two adjacent trenches. In an embodiment, the trenches are formed by a first etching process to obtain one or more elongated structures 403, a second etching is then performed around the one or more elongated structures 403 to form trenches 405. Each of the trenches surrounds each one of the elongated structures. For the sake of brevity, only one elongated silicon structure 403 having trench 405 disposed along a periphery of the base of the elongated silicon structure will be used to discuss the process according to an embodiment of the present invention. However, it is understood that the process can be applied to more than one elongated silicon structure.

After the formation of trenches 405, a region of the silicon substrate below the elongated structure is doped with a first type impurity. One or more etch processes are then performed on the elongated silicon structure to obtain an arched profile on the top portion of the elongated structure, as shown in FIG. 32. The etch process may include thermal oxidation followed by a diluted HF wet etching as described in the process step 2630 described in sections above. The etch process may be repeated multiple times until an arched top portion is obtained. Thereafter, a hydrogen annealing is carried out at a temperature ranging from about 800° C. to 1200° C. to turn the arched top portion of the elongated structure to a semi-circular shape and to smooth out the surface of the elongated structure. A more detail description has been provided in sections above and will not be repeated herein for the sake of brevity.

As illustrated in FIG. 33, the trench is then filled with an insulating material to form shallow insulating structures 401. The insulating material can be silicon oxide or silicon nitride. Thereafter, the elongated structure 403 is doped with dopants using an implanting process. The dopants have a polarity different from the polarity of the first type impurity. A gate dielectric layer 404 is formed on the elongated structure.

Thereafter, a conductive layer is formed on the gate dielectric layer. In an embodiment, the conductive layer and the gate dielectric layer are patterned to form a gate electrode. A gate voltage may be applied to the gate electrode for controlling a current flow across the channel of the device. In an embodiment, the elongated silicon structure is associated with a finFET transistor device. The finFET transistor device comprises a gate electrode, a source region, a drain region, and a channel region below the gate electrode and between the source and drain regions.

In a PMOS transistor device, the fin structure is doped with a p-type impurity. Because the fin structure is formed from the elongated silicon structure, the source, drain, and channel regions have the same dopant polarity and the same dopant concentration. Similarly, in an NMOS, the fin structure is doped an n-type impurity, the source, drain, and channel regions have the same dopant polarity and the same dopant concentration.

It is also understood that the examples and embodiments described herein are for illustrative purposes only and that various modifications or changes in light thereof will be suggested to persons skilled in the art and are to be included within the spirit and purview of this application and scope of the applied claims. 

What is claimed is:
 1. A method for fabricating a transistor device, the method comprising: providing a semiconductor substrate; forming an elongated semiconductor structure overlying a region of the semiconductor substrate along a first direction, the elongated semiconductor structure having a rectangular cross-section; etching the elongated semiconductor structure to form an arched cross-section top portion and a rectangular cross-section bottom portion; implanting dopants of a first-type impurity in the elongated semiconductor structure; forming a dielectric layer on the elongated semiconductor structure; depositing a conductive material on the dielectric layer; and masking and etching the conductive material and the dielectric layer to form a gate electrode for the transistor device.
 2. The method of claim 1, wherein the semiconductor substrate comprises: a silicon substrate; a silicon oxide layer overlying the silicon substrate; and a silicon layer overlying the silicon oxide layer.
 3. The method of claim 1, further comprising: implanting dopants of a second-type impurity in the region of the semiconductor substrate the elongated semiconductor structure prior to forming the elongated semiconductor structure.
 4. The method of claim 1 further comprising: etching around the elongated semiconductor structure to form a trench at a periphery of a low end of the rectangular cross-section; filling the trench with an insulating material to form an insulating structure, wherein the insulating structure is formed prior to implanting dopants of the first-type impurity in the elongated semiconductor structure.
 5. The method of claim 4, wherein the arched cross-section top portion comprises a semi-circular shape having a radius and the rectangular cross-section bottom portion comprises a height, the height being equal to or greater than the radius.
 6. The method of claim 1 wherein the elongated semiconductor structure comprises silicon, germanium, silicon germanium, or other III or V group semiconductor materials.
 7. The method of claim 1, wherein etching the elongated semiconductor structure comprises: oxidizing the elongated semiconductor structure; and removing oxidized portions of the elongated semiconductor structure.
 8. The method of claim 7 further comprising annealing the elongated semiconductor structure.
 9. The method of claim 1, wherein etching the elongated semiconductor structure comprises: thermally oxidizing the elongated semiconductor structure comprising a silicon material, and removing the oxidized material using a diluted hydrofluoric acid.
 10. The method of claim 9 further comprising: hydrogen annealing at a temperature range from about 1000° C. to about 1200° C. and for a time period of about 5 minutes to 30 minutes. 